For the batch processing of wafers in vertical furnaces at very high temperatures it is proposed in U.S. provisional application No. 60/365,122, filed Mar. 15, 2002 (incorporated herein by reference) to support wafers, accommodated in a wafer boat, by susceptor plates. A susceptor plate supports a wafer over its entire surface to prevent slip of the wafer during the high temperature processing. Loading/unloading of wafers into a wafer boat with susceptor plates can be carried out according to the loading/unloading procedure described in the above reference application. The '122 application proposes to have the susceptor plates removably accommodated in the wafer boat. In the loading procedure, a wafer is transported by a wafer handling robot to a transfer station where it is placed onto a susceptor plate. The susceptor plate, accommodating the wafer on its upper surface, is transported from the transfer station to the wafer boat by the wafer handling robot. The susceptor plates are contacted by the same generic end effector as the wafers.
Susceptor plates used for high temperature applications are generally made from silicon carbide (SiC). A common problem of a SiC-object is its surface roughness. The contact surface of the end effector can be damaged by the rough surface of the susceptor plate and subsequently is a potential source for damaging wafers handled by this end effector.
One way to avoid damage of the wafer is to use two end effectors instead of one end effector. The first end effector is used to handle semiconductor wafers and the second end effector is used to handle susceptor plates. Extra space inside the semiconductor processing tool would be needed to accommodate a second end effector. The space necessary is not always available and, when available, could be more productively employed in other fashions. Furthermore, the logistics of such a solution are very complex.
Hence, it is an object of the present invention to provide a single end effector that is able to handle susceptor plates and wafers, desirably without the susceptor plate ever touching the contact surface of said end effector on which the wafer is to be supported.
According to one aspect of the invention, an end effector is provided with a first contact surface to support a semiconductor wafer and a second contact surface, displaced relative to the first contact surface, to support a susceptor plate. Preferably, the first contact surface is on a first vertical level and the second contact surface is on a second vertical level.
According to one embodiment, the second surface (for supporting the susceptor plate) is positioned below the first surface. The susceptor plate is provided with a recess in an underside thereof, configured to prevent contact with said first contact surface while the susceptor plate is supported by the second contact surface. The recess faces the first contact surface and has a diameter significantly smaller than the outer diameter of the second contact surface but a large enough to accommodate the first contact surface. The depth of the recess is such that no contact of the susceptor plate is made with the first contact surface while it is supported by the second contact surface.
In another embodiment, an edge support end effector supports a disk-like object at a contact surface at the edge of the object. With this edge support end effector, a wafer is supported at a first, lower level and a susceptor plate with a larger diameter than the wafer is supported at a second higher level. The contact surface can be upwardly and outwardly sloped so that the object to be supported is only contacted at its outer edge. In another embodiment, the contact surface can also comprise steps.